1. Field of the Invention
The present invention generally relates to a cellular communication system, and in particular to a method for monitoring radio channels for channel switching in a mobile communication terminal such as a cellular telephone.
2. Description of the Related Art
In cellular communications, it is necessary to transfer a connection from one cell to another without interruption. For this end, a mobile terminal monitors radio channels of adjacent cells at all times so that, when the condition of a radio channel being used becomes worse and that of an adjacent radio channel becomes better, the mobile terminal changes the communication channel to the adjacent radio channel having a better radio condition.
One method of monitoring radio channels when the mobile terminal is moving has been disclosed in Japanese Patent Unexamined Publication No. 3-104330. In this method, the duration of measuring the strength level of a received signal is varied according to a velocity of the moving mobile terminal. As shown in FIG. 1, a mobile terminal has a transmission and reception system including an antenna 10, a radio transmitter-receiver 11, and a modulator-demodulator 12. The mobile terminal is further provided with a fading frequency detector 13, a received level detector 14, and a controller 15.
The fading frequency detector 13 detector facing frequency which is used to estimate a velocity of the mobile terminal by the controller 15. More specifically, the fading frequency detector 13, receiving an intermediate frequency signal from the radio transmitter-receiver 11, detects the envelope of a received signal strength. By comparing it with a predetermined household level, the fading frequency detector 13 counts the number of times the envelope has decreased below the predetermined threshold level for a predetermined time period to produce the fading frequency. When the fading frequency is low, it is estimated that the mobile terminal is moving at a relatively low velocity. Conversely, when the fading frequency is high, it is estimated that the mobile terminal is moving at a relatively high velocity.
Based on the estimated velocity, the duration of measuring the strength level of a received signal is changed. At a low velocity, the measuring duration is long enough to produce a significant averaged level of the strength level of the received signal. On the other hand, at a high velocity, the measuring duration is short so as to rapidly determine whether to transfer from one cell to another.
However, the velocity estimation method mentioned above cannot assure continued accuracy because the received signal strength varies due to not only the movement velocity of the mobile terminal but also multipath fading which is a characterization of radio propagation. More specifically, variations in received signal strength due to the multipath fading cause the fading frequency to change, resulting in reduced accuracy of the velocity estimation based on the fading frequency. This causes undesirable channel switching in a radio zone or a delay in transferring from a radio zone to another. In this manner, according to the conventional control method, it is difficult to transfer a connection with reliability. Further, since the measuring duration is relatively long at a low velocity, the power consumption is increased.